One well-known method for the exposure of photographic photosensitive elements is an image forming method of the scanner system comprising the steps of scanning an original to produce image signals, subjecting a photographic silver halide photosensitive element to exposure in accordance with the image signals, and forming a negative or positive image corresponding to the image of the original.
There is a desire to have a procedure of providing outputs of a scanner to a film and directly printing on a printing plate without a transfer step as well as a scanner photosensitive element having a ultrahigh contrast with respect to a scanner light source having a soft beam profile.
There are known a number of photosensitive elements having a photosensitive layer on a support wherein images are formed by imagewise exposure. Among these, a technique of forming images through heat development is known as a system capable of simplifying image forming means and contributing to environmental protection.
From the contemporary standpoints of environmental protection and space saving, it is strongly desired in the photomechanical process field to reduce the quantity of spent solution. Needed in this regard is a technology relating to photothermographic elements for use in photomechanical process which can be effectively exposed by means of laser scanners or laser image setters and produce distinct black images having a high resolution and sharpness. These photothermographic elements offer the customer a simple thermographic system that eliminates the need for solution type chemical reagents and is not detrimental to the environment.
The technology of forming images through heat development is disclosed, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075, D. Morgan and B. Shely, "Thermally Processed Silver Systems" in "Imaging Processes and Materials," Neblette, 8th Ed., Sturge, V. Walworth and A. Shepp Ed., page 2, 1969. These photothermographic elements generally contain a reducible non-photosensitive silver source (e.g., organic silver salt), a catalytic amount of a photocatalyst (e.g., silver halide), and a reducing agent for silver, typically dispersed in an organic binder matrix. Photothermographic elements are stable at room temperature. When they are heated at an elevated temperature (e.g., 80.degree. C. or higher) after exposure, a redox reaction takes place between the reducible silver source (functioning as an oxidizing agent) and the reducing agent to form silver. This redox reaction is promoted by the catalysis of a latent image produced by exposure. Silver formed by reaction of the reducible silver salt in exposed regions provides black images in contrast to unexposed regions, forming an image.
Photothermographic elements of this type are well known in the art. In most of these elements, photosensitive layers are formed by applying coating solutions based on organic solvents such as toluene, methyl ethyl ketone (MEK) and methanol, followed by drying. The use of organic solvents is not only harmful to workers in the manufacturing procedure, but also disadvantageous because of the cost for recovery and disposal of the solvents.
To eliminate these concerns, it is contemplated to form photosensitive layers using coating solutions based on water solvent. Such photosensitive layers are sometimes referred to as "aqueous photosensitive layers," hereinafter. For example, JP-A 52626/1974 and 116144/1978 disclose the use of gelatin as the binder. JP-A 151138/1975 discloses polyvinyl alcohol as the binder. Further, JP-A 61747/1985 discloses a combined use of gelatin and polyvinyl alcohol. Besides, JP-A 28737/1983 discloses a photosensitive layer containing water-soluble polyvinyl acetal as the binder.
It is true that the use of these binders has great environmental and economical advantages in that photosensitive layers can be formed using coating solutions based on water solvent.
However, the use of such polymers as gelatin, polyvinyl alcohol and water-soluble polyacetal as the binder has the following drawbacks. These binders are so poorly compatible with the organic silver salt that they may form coatings which are practically unacceptable in coating surface quality. The silver tone of developed areas becomes far apart from the inherently desirable black color, for example, brown or yellow. There result developed areas of a low blackening density and unexposed areas of a high density. The elements are thus extremely low in commodity value.
There is a desire to have a photothermographic element which is an aqueous photosensitive element advantageous from the environment and economical aspects and has good coating surface quality, improved silver tone upon development, and satisfactory photographic properties.
The use of such aqueous photosensitive layers enables application of the techniques used in conventional well-known photographic silver halide photosensitive materials. More particularly, it becomes possible to preform a photosensitive silver halide emulsion having desired properties and then admix it with an organic silver salt. The freedom of design is significantly improved over the method of preparing photosensitive silver halide in an organic solvent system.
On the other hand, the recent rapid progress of semiconductor laser technology has made it possible to reduce the size of medical image output devices. As a matter of course, there were developed techniques relating to infrared-sensitive photothermal silver halide photographic material which can utilize a semiconductor laser as a light source. The spectral sensitization technique is disclosed, for example, in JP-B 10391/1991 and 52387/1994, JP-A 341432/1993, 194781/1994, and 301141/1994. The antihalation technique is disclosed, for example, in JP-A 13295/1995 and U.S. Pat. No. 5,380,635. Since the infrared exposure system permits the visible absorption of sensitizing dyes and antihalation dyes to be considerably reduced, a substantially colorless photosensitive material can be readily produced.
Since spectral sensitizing dyes capable of absorbing infrared radiation, however, generally have a high reducing power due to a high HOMO (highest occupied molecular orbital), they tend to reduce silver ions in photosensitive materials to exacerbate the fog thereof. In particular, these photosensitive materials experience a substantial change of performance during storage under hot humid conditions and long-term storage. If dyes having a low HOMO are used for preventing deterioration of storage stability, spectral sensitization efficiency and sensitivity become low because their LUMO (lowest unoccupied molecular orbital) is relatively low. These problems relating to sensitivity, storage stability, and performance variation arise not only with wet photographic photosensitive materials, but more outstandingly with photothermographic materials.